Coal gangue is a major by-product of coal processing and beneficiation. It contains approximately 18% to 40% alumina and 40 to 65% silica. High alumina-content (≥30%) coal gangue has become a promising substitute for bauxite, a major aluminium source, due to the decrease of bauxite reserves and degradation of ore bodies. Many attempts have been made to extract aluminium and silicon products, as well as valuable metals such as gallium and titanium from coal gangue (Qian and Li 2015; Geng, et al., 2012; Zhao 2014). For example, aluminium products have been produced from coal gangue by acid leaching. However, iron(III) is the third most abundant element in coal gangue, occupying 2 to 10% in total and the iron in the raw material is brought into leaching solutions during the leaching process and this contaminates and stains resulting products. Therefore, the removal of iron from coal gangue leaching solutions is essential in order to obtain commercially acceptable aluminium, or other valuable metal products with high purity from these solutions.
At present, iron is usually removed from leach solutions by precipitation (Chang et al. 2010, Swarnkar et al. 1996). However, this method has some drawbacks, such as low selectivity due to co-precipitation and generation of fine precipitates which are difficult to remove from the leachate. Furthermore, the precipitates, commonly containing heavy metal ions, are of environmental concern. In addition, their disposal in controlled ponds is becoming more and more expensive due to strict environmental regulations.
Solvent extraction can separate, purify and concentrate metal ions, leading to subsequent production of pure products. It is a process that allows the separation of two or more components due to their unequal solubilities in two immiscible liquid phases. It is an important method for metal ion separation and also a commonly used industrial technique of major significance such as continuous operation mode, simple equipment, as well as high throughput (Tian, Li et al. 2010). In solvent extraction, dissolved metals are selectively separated from other components from an aqueous solution by an extraction agent. Traditionally, solvent extraction involves water-immiscible organic solvents, many of which are flammable, volatile or toxic. This gives rise to environmental and workplace hazards.
There is thus a need to provide processes that can be used to selectively remove iron from feed stocks containing other non-ferric metals, such as aluminium.